Key Engineering Materials Vols. 353-358

Abstract: Magnetic camera consists of magnetic source, arrayed small magnetic sensors, magnetic lens, analog-to-digital converter and interface, computer and monitor. The quantitative magnetic field around crack and its processed results could be obtained by using magnetic camera, and the crack could be inspected and evaluated quantitatively. In addition, the magnetic camera has to uphold with large lift-off to protect sensors from weak environment such as high temperature and mechanical vibration. However, the sensor sensitivity would be decreased when the lift-off was increased. Correspondingly, the improved techniques are necessary for increasing sensitivity of magnetic camera and probability of crack detection at the large lift-off. This paper proposes an image processing method, which separates a global full scale to the several regions and repeats shadings in each region, to increase a crack detection probability in the magnetic camera images such as ∂B/∂x and ∂2B/∂x∂y.

Abstract: The yoke type magnetization coil and cross type magnetizer as a magnetic source, which induce a static magnetic field on a specimen, have been used to detect cracks on a ferromagnetic material. However, the cracks on paramagnetic materials such as aluminum alloy and stainless steel cannot be detected by using a static magnetic source. This paper proposes a magnetic source, which uses the induced sheet type current, for the detection of cracks on a paramagnetic material. The sheet type AC current can be induced by using a primary coil and core. And the copper film, which includes a sheet type current, is positioned on the specimen. Eddy currents are induced around a crack on the specimen because the secondary sheet type current can be induced on the specimen surface. The signal processing electrical circuits, which measure a Hall sensor peak output with the eddy current frequency, are developed and discussed. Also the validity of the proposed signal processing circuit is verified by detection of slit type cracks and a fatigue crack, which are introduced on aluminum alloy.

Abstract: The magnetic field distribution around a crack can be calculated more easily and quickly by using a dipole model than finite element method (FEM). This paper reports the development of numerical analysis software that uses an improved dipole model to analyze the magnetic field around cracks. The preprocessor in this software includes the crack formation software, which can distribute the magnetic charge per unit area, m, on the crack section area. Also the lift-off, measurement area and sensor interval, and magnetization direction can be considered in the preprocessor. Also, the postprocessor presents functions, such as the natural magnetic field distribution and ∂B/∂x, ∂B/∂y, as results. Also, the physical characteristics of the magnetic optical sensor and the Hall sensor are included in the postprocessor, and the magnetic field distribution can be changed to optical intensity and electrical signal distribution. The experiment results, which are obtained by using the magnetic camera on the crack, are compared with analysis results obtained by using the dipole model analysis software.

Abstract: Series of slow strain rate tests (SSRT) were conducted on moderately sensitized austenitic stainless steel in diluted aqueous solution of sodium thiosulfate at ambient temperature and pressure. Only a small area of the test piece was exposed to test solution and electrochemical potential transients obtained during the straining was used to detect an initiation or a precursor event of stress corrosion crack (SCC). Tests were stopped intermittently after getting potential transients, so as not to allow crack to propagate and to observe the morphology of very initial stage of SCC initiation site. Visual observation by scanning electron microscope (SEM) before and after the test shows direct correlation between electrochemical transient of certain characteristic to the crack initiation. SEM observation shows δ- ferrite in the alloy as one of the preferential SCC initiation site for typical electrochemical transient. Attempt was made to calculate the charge associated with crack initiation event from potential transient using dummy anode test.

Abstract: In order to assure the reliability of advanced gas turbine systems, it is very important to evaluate the damage of high temperature materials such as Ni-based superalloys under creep and fatigue conditions quantitatively. The refractive index of the gamma-prime phase is found to be smaller than that of the gamma phase in the Ni-based superalloy, when the wavelength of an irradiated laser beam is shorter than 500 nm. Therefore, it is possible to evaluate the creep damage of this material quantitatively and non-destructively by observing the change of the micro texture in a grain (rafting) using a scanning laser microscope.

Abstract: Monotonic and cyclic loadings were subjected to electrodeposited copper foils (thickness is 8 and 20 μm), and the deformation behavior was observed. In-situ X-ray stress measurement was carried out under monotonic loading. The tensile strength of 8 μm foil was higher than that of 20 μm foil. On the other hand, the elongation of 8 μm foil was smaller. When the plastic deformation occurred, difference between the X-ray stress and the applied stress became large. The difference of 20 μm foil was larger than that of 8 μm foil. Fatigue strength of 8 μm foil was also higher than that of 20 μm foil. The value of the full width at half maximum, FWHM, increased dramatically at the first cycle, and then the value became nearly constant. Just before fracture, the value increased again. The change in FWHM corresponded to the change in the accumulated ratchet strain.

Abstract: AE method is a well-known technique for in-situ monitoring of damage behavior by attached piezoelectric transducer. However, this conventional detection of AE signals has certain limitations. In recent years, numerous efforts have addressed the substitution of laser-based techniques for ultrasonic nondestructive evaluation in place of conventional piezoelectric transducers. Especially, a laser interferometer can be used to measure a displacement or velocity at materials surface using Doppler-shift. However, there are few reports referring to the detection of AE signals in the practical materials and testing because of the difficulty of experiments. We developed the AE measurement system with laser interferometer to apply this technique to microcrack evaluation and reported the quantitative AE analysis in various materials. This paper demonstrates AE results from thermal spray coatings at elevated temperature.

Abstract: This study dealt with the corrosion resistance for the carbon steels under a pressurized water atmosphere at the elevated temperature. The nondestructive test was also used to evaluate the damage degree of corrosion test specimen. The corrosion test for carbon steels was carried out at the temperature of 200 °C under a water pressure of 10 MPa. The corrosion time for carbon steel was changed up to 20 weeks. The strength of carbon steel by the degree of corrosion was investigated by a tensile test. The carbon steel showed an average tensile strength of about 500 MPa after the corrosion period of 20 weeks, accompanying the weight loss of about 2.5 %. The attenuation coefficient of ultrasonic wave can be utilized as useful parameters to inspect the corrosion damages of carbon steels.

Abstract: The nuclear power plant has lots of pipes that the fluid of high temperature and high pressure flows. Among the pipe materials used at secondary circuit of the power plant the carbon steels are sensitive to corrosion due to their material properties. In this study, both ultrasonic test and acoustic emission test were used to study the corrosion effect for the carbon steel pipe nondestructively. The carbon steel specimens were in the pipe under 473K temperatures and 10MPa pressure conditions for corrosion processing. According to the degree of corrosion the strength of the specimen was evaluated, and the thickness of the corrosion specimens was also measured by using the ultrasonic wave. The experimental results showed that the attenuation factor was also increased as a depth of corrosion increased. The measured depth of the real corrosion by ultrasonic test shows the good agreement with that by an optical microscope. In order to understand the corrosion effect for the failure mechanism of carbon steel, a failure test on the specimen with various corrosion conditions was performed. An acoustic emission technique was also used to evaluate the degree of damage of corrosion specimen in real time. Acoustic emission technique is proved a useful method for on-line monitoring the microscopic failure mechanism and the damage location for the structures.